Operating in power save mode

ABSTRACT

A method for operating in a power save mode for a wireless local area network and a device using the same are provided. After waking up, a device receives a frame via a wireless medium to determine that the wireless medium is busy. If a decoding of the frame in a medium access control (MAC) layer is unsuccessful but a decoding of the frame in a physical layer is successful, the device determines an Interframe space (IFS) to defer an access of the wireless medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation Application (BypassContinuation Application) of a currently pending internationalapplication No. PCT/IB2015/001833 having an international filing date of26 Jun. 2015 and designating the United States, the internationalapplication claiming priority to the following earlier filed Koreanpatent application No. 10-2014-0080174 filed on Jun. 27, 2014. Theentire contents of the aforesaid international application and theafore-listed Korean patent applications are incorporated herein byreference. The applicant claims the benefit of and claims priory hereinto the aforesaid international application and the afore-listed Koreanpatent applications and their filing dates and priority dates.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method for operating in a power save mode for awireless local area network and a device using the same.

Related Art

Institute of Electrical and Electronics Engineers (IEEE) 802.11nstandard established in 2009 provides a transfer rate of up to 600 Mbpsat a frequency band of 2.4 GHz or 5 GHz on the basis of Multiple InputMultiple Output (MIMO) technique.

IEEE 802.11ac standard established in 2013 aims to provide a throughputgreater than or equal to 1 Gbps utilizing Medium Access Control (MAC)Service Access Point (SAP) layer scheme at a frequency band less than orequal to 6 GHz. A system supporting IEEE 802.11ac standard is referredto as a Very High Throughput (VHT) system.

There are continuing efforts to implement more effective Wireless LocalArea Network (WLAN) technologies in increasingly congested environments.

SUMMARY OF THE INVENTION

The present invention provides a method for operating in a power savemode for a wireless local area network.

The present invention also provides a device for operating in a powersave mode for a wireless local area network.

In an aspect, a method for operating in a power save mode for a wirelesslocal area network is provided. The method includes transitioning, by astation, from a doze state to an awake state, receiving, by the stationin the awake state, a frame via a wireless medium to determine that thewireless medium is busy, and if a decoding of the frame in a mediumaccess control (MAC) layer is unsuccessful but a decoding of the framein a physical layer is successful, determining, by the station in theawake state, an Interframe space (IFS) to defer an access of thewireless medium.

The IFS may be a Distributed coordination function (DCF) InterframeSpace (DIFS) if the type of the response frame is identified.

The IFS may be an Extended Interframe space (EIFS) that is longer thanthe DIFS if the type of the response frame is not identified.

A type of the response frame is identified by a signal field in aphysical header of the frame.

The type of the response frame may be identified if the signal fieldincludes a response indication indicating a type of an expectedresponse.

In another aspect, a device configured for operating in a power savemode for a wireless local area network is provided. The device includesa radio frequency module configured to transmit and receive radiosignals, and a processor operatively coupled with the radio frequencymodule and configured to transition from a doze state to an awake state,instruct the radio frequency module to receive a frame via a wirelessmedium to determine that the wireless medium is busy, and determine anInterframe space (IFS) to defer an access of the wireless medium if adecoding of the frame in a medium access control (MAC) layer isunsuccessful but a decoding of the frame in a physical layer issuccessful.

Various kinds of data frames can be aggregated. A power consumption of astation can be decreased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows PPDU formats used by the legacy system.

FIG. 2 shows an HEW PPDU format according to an embodiment of thepresent invention.

FIG. 3 shows constellation phases for the conventional PPDU.

FIG. 4 shows constellation phases for a proposed HEW PPDU.

FIG. 5 shows an example of a Medium Access Control (MAC) frame formatbased on the conventional IEEE 802.11.

FIG. 6 shows another example of a MAC frame format based on theconventional IEEE 802.11.

FIG. 7 shows an A-MPDU format according to an aggregation scheme for anMPDU.

FIG. 8 shows a frame format according to an embodiment of the presentinvention.

FIG. 9 shows an A-MPDU format having a PV0 Null Data frame.

FIG. 10 is a block diagram of an STA according to an embodiment of thepresent invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The proposed wireless local area network (WLAN) system may operate at aband less than or equal to 6 GHz or at a band of 60 GHz. The operatingband less than or equal to 6 GHz may include at least one of 2.4 GHz and5 GHz.

For clarity, a system complying with the Institute of Electrical andElectronics Engineers (IEEE) 802.11 a/g standard is referred to as anon-High Throughput (non-HT) system, a system complying with the IEEE802.11n standard is referred to as a High Throughput (HT) system, and asystem complying with IEEE 802.11 ac standard is referred to as a VeryHigh Throughput (VHT) system. In comparison thereto, a WLAN systemcomplying with the proposed method is referred to as a High EfficiencyWLAN (HEW) system. A WLAN system supporting systems used before the HEWsystem is released is referred to as a legacy system. The HEW system mayinclude an HEW Station (STA) and an HEW Access Point (AP). The term HEWis only for the purpose of distinguishing from the conventional WLAN,and there is no restriction thereon. The HEW system may support IEEE802.11/a/g/n/ac by providing backward compatibility in addition to theproposed method.

Hereinafter, unless a function of a station (STA) is additionallydistinguished from a function of an Access Point (AP), the STA mayinclude a non-AP STA and/or the AP. When it is described as an STA-to-APcommunication, the STA may be expressed as the non-AP STA, and maycorrespond to communication between the non-AP STA and the AP. When itis described as STA-to-STA communication or when a function of the AP isnot additionally required, the STA may be the non-AP STA or the AP.

A Physical layer Protocol Data unit (PPDU) is a data unit for datatransmission.

FIG. 1 shows PPDU formats used by the legacy system.

A non-HT PPDU supporting IEEE 802.11a/g includes a Legacy-Short TrainingField (L-STF), a Legacy-Long Training Field (L-LTF), and a Legacy-Signal(L-SIG).

An HT PPDU supporting IEEE 802.11n includes a HT-SIG, a HT-STF, and aHT-LTF after the L-SIG.

A VHT PPDU supporting IEEE 802.11ac includes a VHT-SIG-A, a VHT-STF, aVHT-LTF, and a VHT-SIG-B after the L-SIG.

FIG. 2 shows an HEW PPDU format according to an embodiment of thepresent invention.

An L-STF may be used for frame detection, Automatic Gain Control (AGC),diversity detection, and coarse frequency/time synchronization.

An L-LTF may be used for fine frequency/time synchronization and channelestimation.

An L-SIG may include information indicating a total length of acorresponding PPDU (or information indicating a transmission time of aphysical layer protocol service unit (PSDU)).

The L-STF, the L-LTF and the L-SIG may be identical to L-STF, L-LTF andL-SIG of the VHT system. The L-STF, the L-LTF and the L-SIG may bereferred to as a legacy portion. The L-STF, the L-LTF, and the L-SIG maybe transmitted in at least one Orthogonal Frequency DivisionMultiplexing (OFDM) symbol generated on the basis of 64-points FastFourier Transform (FFT) (or 64 subcarriers) in each 20 MHz channel. For20 MHz transmission, the legacy portion may be generated by performingan inverse Discrete Fourier Transform (IDFT) with 64 FFT points. For 40MHz transmission, the legacy portion may be generated by performing anIDFT with 128 FFT points. For 80 MHz transmission, the legacy portionmay be generated by performing an IDFT with 512 FFT points.

A HEW-SIGA may include common control information commonly received byan STA which receives a PPDU. The HEW-SIGA may be transmitted in 2 OFDMsymbols or 3 OFDM symbols.

The following table exemplifies information included in the HEW-SIGA. Afield name or the number of bits is for exemplary purposes only.

TABLE 1 Field Bits Description Bandwidth 2 Set to 0 for 20 MHz, 1 for 40MHz, 2 for 80 MHz, 3 for 160 MHz and 80 + 80 MHz mode STBC 1 Set to 1 ifall streams use STBC, otherwise set to 0. When STBC bit is 1, an oddnumber of space time streams per user is not allowed. Group ID 6 Set tothe value of the TXVECTOR parameter GROUP_ID. A value of 0 or 63indicates a HEW SU PPDU; otherwise, indicates a HEW MU PPDU.Nsts/Partial AID 12 For MU: 3 bits/user with maximum of 4 users  Set to0 for 0 space time streams  Set to 1 for 1 space time stream  Set to 2for 2 space time streams  Set to 3 for 3 space time streams  Set to 4for 4 space time streams Otherwise: first 3 bits contain streamallocation for SU, set to 0 for 1 space time stream, set to 1 for 2space time streams, etcetera up to 8 streams. Remaining 9 bits containpartial association identifier (AID). No TXOP PS 1 Set to 1 to indicatethat TXOP PS is not allowed. Set to 0 to indicate that TXOP PS isallowed. Set to the same value in all PPDUs in downlink MU TXOP. GI(Guard 2 Set B0 to 0 for Long GI, set to 1 for Short GI. Set B1 to 1when interval) Short GI. Coding 2 For SU:  Set B2 to 0 for BCC, set to 1for LDPC For MU:  Set B2 to 0 for BCC, set to 1 for LDPC for 1st user Ifuser 1 has 0 Nsts value, then B2 is reserved and set to 1 MCS 4 ForSU/Broadcast/Multicast: Modulation and coding scheme (MCS) index For MU:  B1: Set to 0 for BCC, 1 for LDPC for the 2nd user   B2: Set to 0 forBCC, 1 for LDPC for the 3rd user   B3: Set to 0 for BCC, 1 for LDPC forthe 4th user If user 2, 3, or 4 has 0 Nsts value, then corresponding bitis reserved and set to 1 SU- 1 Set to 1 when packet is a SU-beamformedpacket Beamformed Set to 0 otherwise For MU: Reserved, set to 1 CRC 8Tail 6 All zeros

A HEW-STF may be used to improve an AGC estimation in an MIMOtransmission.

A HEW-LTF may be used to estimate a MIMO channel. The HEW-LTF may startat the same point of time and may end at the same point of time acrossall users.

A HEW-SIGB may include user-specific information required for each STAto receive its PSDU. For example, the HEW-SIGB may include informationregarding a length of a corresponding PSDU and/or a bandwidth or channelin which the PSDU for a corresponding receiver is transmitted.

A data portion may include at least one PSDU. The position of theHEW-SIGB is illustration purpose only. The HEW-SIGB may be followed bythe data portion. The HEW-SIGB may be followed by the HEW-STF or theHEW-LTF.

In the proposed PPDU format, the number of OFDM subcarriers may beincreased per unit frequency. The number of OFDM subcarriers mayincrease K-times by increasing FFT size. K may be 2, 4, or 8. Thisincrease may be accomplished via downclocking (e.g. using a larger FFTsize with a same sampling rate).

For example, K=4 downclocking is assumed. As for the legacy portion, 64FFT is used in a 20 MHz channel, 128 FFT is used in a 40 MHz channel,and 256 FFT is used in an 80 MHz channel. As for a HEW portion using thelarger FFT size, 256 FFT is used in a 20 MHz channel, 512 FFT is used ina 40 MHz channel, and 1024 FFT is used in an 80 MHz channel. TheHEW-SIGA may have same FFT size as the legacy portion. The HEW portionmay have larger FFT size than the legacy portion.

The PPDU is generated by performing IDFT with two different FFT sizes.The PPDU may include a first part with a first FFT size and a secondpart with a second FFT size. The first part may include at least one ofthe L-STF, the L-LTF, the L-SIG and the HEW-SIGA. The second part mayinclude at least one of the HEW-STF, the HEW-LTF and the data portion.The HEW-SIGB may be included in the first part or in the second part.

When an FFT size is increased, an OFDM subcarrier spacing is decreasedand thus the number of OFDM subcarriers per unit frequency is increased,but an OFDM symbol duration is increased. A guard interval (GI) (or alsoreferred to as a Cyclic Prefix (CP) length) of the OFDM symbol time canbe decreased when the FFT size is increased.

If the number of OFDM subcarriers per unit frequency is increased, alegacy STA supporting the conventional IEEE 80.2.11a/g/n/ac cannotdecode a corresponding PPDU. In order for the legacy STA and an HEW STAto co-exist, L-STF, L-LTF, and L-SIG are transmitted through 64 FFT in a20 MHz channel so that the legacy STA can receive the L-STF, the L-LTF,and the L-SIG. For example, the L-SIG is transmitted in a single OFDMsymbol, a symbol time of the single OFDM symbol is 4 micro seconds (us),and the GI is 0.8 us.

Although the HEW-SIGA includes information required to decode an HEWPPDU by the HEW STA, the HEW-SIGA may be transmitted through 64 FFT inan 20 MHz channel so that it can be received by both of the legacy STAand the HEW STA. This is to allow the HEW STA to receive not only theHEW PPDU but also the conventional non-HT/HT/VHT PPDU.

FIG. 3 shows constellation phases for the conventional PPDU.

To identify a format of a PPDU, a phase of a constellation for two OFDMsymbols transmitted after L-STF, L-LTF, and L-SIG is used.

A ‘first OFDM symbol’ is an OFDM symbol first appeared after the L-SIG.A ‘second OFDM symbol’ is an OFDM symbol subsequent to the first OFDMsymbol.

In a non-HT PPDU, the same phase of the constellation is used in the 1stOFDM symbol and the 2nd OFDM symbol. Binary Phase Shift Keying (BPSK) isused in both of the 1st OFMD symbol and the 2nd OFDM symbol.

In an HT PPDU, although the same phase of the constellation is used inthe 1st OFDM symbol and the 2nd OFDM symbol, the constellation rotatesby 90 degrees in a counterclockwise direction with respect to the phaseused in the non-HT PPDU. A modulation scheme having a constellationwhich rotates by 90 degrees is called Quadrature Binary Phase ShiftKeying (QBPSK).

In a VHT PPDU, a constellation of the first OFDM symbol is identical tothat of the non-HT PPDU, whereas a constellation of the second OFDMsymbol is identical to that of the HT PPDU. The constellation of secondOFDM symbol rotates 90 degrees in a counterclockwise direction withrespect to the 1st OFDM symbol. The first OFDM symbol uses BPSKmodulation, and the 2nd OFDM symbol uses QBPSK modulation. SinceVHT-SIG-A is transmitted after L-SIG and the VHT-SIG-A is transmitted intwo OFDM symbols, the first OFDM symbol and the second OFDM symbol areused to transmit the VHT-SIG-A.

FIG. 4 shows constellation phases for a proposed HEW PPDU.

To distinguish from a non-HT/HT/VHT PPDU, a constellation of at leastone OFDM symbol transmitted after L-SIG can be used.

Just like the non-HT PPDU, a first OFDM symbol and a second OFDM symbolof the HEW PPDU have the same constellation phase. A BPSK modulation maybe used for the first OFDM symbol and the second OFDM symbol. The STAcan differentiate the HEW PPDU and HT/VHT PPDUs.

In an embodiment, to differentiate the HEW PPDU and the non-HT PPDU, theconstellation of a third OFDM symbol can be utilized. The constellationof the third OFDM symbol may rotate by 90 degrees in a counterclockwisedirection with respect to the second OFDM symbol. The first and secondOFDM symbols may use BPSK modulation, but the third OFDM symbol may useQBPSK modulation.

In another embodiment, the HEW-SIGA may provide an indication about theformat of the PPDU. The indication may indicate whether the format ofthe PPDU is a HEW PPDU. The HEW-SIGA may provide an indication about ause of orthogonal frequency division multiple access (OFDMA).

FIG. 5 shows an example of a Medium Access Control (MAC) frame formatbased on the conventional IEEE 802.11.

This frame corresponds to a Protocol Version 0 (PV0) Data frame. The PV0Data frame includes Frame Control, Duration/ID, Address 1 (ReceiverAddress), Address 2 (Transmitter Address), Address 3 (BSSID), SequenceControl, Address 4, Quality-of-Service (QoS) Control, HT Control, MSDU,and Frame Control Sequence (FCS).

The Frame Control field includes Protocol Version, Type, Subtype, To DS,From DS, More Fragment, Retry, Power Management, More Data, ProtectedFrame, and Order.

The Protocol Version may be set to 0 to indicate that a correspondingMAC Protocol Data Unit (MPDU) is a PV0 Data frame. The Type and theSubtype are set to indicate that a corresponding MPDU is a DATA frame,and to specify a detailed type such as QoS data and null data among theData frames. The To DS indicates whether it is transmitted to adistribution system, and the From DS indicates whether it is transmittedfrom the distribution system.

FIG. 6 shows another example of a MAC frame format based on theconventional IEEE 802.11.

This frame corresponds to a Protocol Version 1 (PV1) Data frame. The PV1Data frame includes Frame Control, Address 1 (Receiver Address), Address2 (Transmitter Address), Sequence Control, Address 3, Address 4, MSDU,and FCS.

The Frame Control field of the PV1 Data frame includes Protocol Version,Type, PTID/Subtype, From DS, More Fragment, Power Management, More Data,Protected Frame, End of Service Period, Relayed Frame, and Ack Policy.

The Protocol Version may be set to 1 to indicate that a correspondingMPDU is a PV1 Data frame. The Type and the PTID/Subtype are set toindicate that a corresponding MPDU is a DATA frame, and to specify adetailed type such as QoS data and null data among the Data frames. TheFrom DS indicates whether it is transmitted from a distribution system.According to the From DS field, contents of Address 1 and Address 2 aredetermined.

Table 2 shows contents included in Address 1, Address 2, Address 3, andAddress 4 according to the From DS.

TABLE 2 From DS field Meaning Use 0 A1 contains the MAC address of theFor frames transmitted by a receiver. non-AP STA to an AP. A2 is anShort ID (2 octets) which contains For frames transmitted from a the AIDof the transmitter non-AP STA to non-AP STA A3 (if present) contains theMAC address (direct link) of the destination. A4 (if present) containsthe MAC address of the source. 1 A1 is an SID (2 octets) which containsthe AP to non-AP STA AID of the receiver. A2 is the MAC address of thetransmitter. A3 (if present) contains the MAC address of thedestination. A4 (if present) contains the MAC address of the source.

Comparing the PV0 Data frame and the PV1 Data frame, the PV1 Data frameis different from the PV0 Data frame in a sense that fields consideredas being unnecessary, for example, duration/ID and QoS, are excludedfrom a MAC header. Therefore, the PV1 Data frame may be called a shortdata frame. If a size of an MSDU is great, the PV0 data frame ispreferably used, and if the size of the MSDU is small, the PV1 dataframe is preferably used to decrease an overhead for the MAC header.

FIG. 7 shows an A-MPDU format according to an aggregation scheme for anMPDU.

Each of a plurality of MPDUs is configured with an aggregated-MPDU(A-MPDU) subframe and is transmitted by being aggregated with one PPDU.

The A-MPDU subframe includes a 4-octet MPDU delimiter, a MPDU, and a Padoctet.

The MPDU delimiter includes EOF, MPDU length, CRC, and DelimiterSignature.

As an HEW MAC format, it is proposed an aggregation scheme for differenttypes of MPDUs, i.e., a PV0 Data frame and a PV1 Data frame.

FIG. 8 shows a frame format according to an embodiment of the presentinvention.

PV0 and PV1 Data frames are aggregated within one A-MPDU frame.

When aggregating the PV0 and PV1 Data frames, there is a need todistinguish the PV0 Data frame and the PV1 Data frame in order todecrease decoding complexity of a receiver STA. It is not preferable toaggregate the frames in a mixed manner such as the PV0 Data frame, thePV1 Data frame, the PV0 Data frame, and the PV1 Data frame.

The PV0 Data frame and the PV1 Data frame may be aggregatedsequentially. This means that the PV1 Data frame is included in anA-MPDU subframe only after the PV0 Data frame. Since more pieces ofinformation are included in the PV0 Data frame, a load of decodingprocessing may be decreased.

In order to aggregate the PV0 and PV1 Data frames, there are severalrestrictions as follows.

First, as shown in FIG. 8, a Traffic Identifier (TID) value may beidentical for both of a PV0 Data frame and a PV1 Data frame to beaggregated. The PV0 Data frame is encoded through a TID subfield (4bits) of a QoS control field of a MAC header, and the PV1 Data frame isencoded through a PTID subframe (3 bits) of a Frame Control (FC) fieldof the MAC header. The PTID implies a Partial TID, and implies lower 3bits among 4 bits of a TID subfield of a QoS control field. It is shownin FIG. 8 that TID and PTID subfields of the PV0 Data frame and PV1 Dataframe included in the A-MPDU have the same Traffic Identifier (TID)value of ‘B’.

Second, the Address 1 and the Address 2 may indicate the same STA. Incase of the PV0 Data frame, the Address 1 includes a receiver STA MACaddress, and the Address 2 includes a transmitter STA MAC address.However, in case of the PV1 Data frame, although the Address 1 indicatesthe receiver STA and the Address 2 indicates the transmitter STA in thesame manner as described above, a short ID value including an AID isused as one of the Address 1 and the Address 2 according to the From DSsubfield of the Frame Control field.

This implies that the receiver STA indicated by the Address 1 withrespect to the PV0 Data frame and the PV1 Data frame may be identicaleven though contents of the Address 1 are different from each other withrespect to the PV0 Data frame and the PV1 Data frame. Also, this impliesthat the transmitter STA indicated by the Address 2 with respect to thePV0 Data frame and the PV1 Data frame may be identical even thoughcontents of the Address 2 are different from each other with respect tothe PV0 Data frame and the PV1 Data frame.

Third, sequence number values of Sequence Control fields for the PV0Data frame and the PV1 Data frame may be managed as one counter. Thisimplies that the PV0 Data frame with SN1 and the PV1 Data frame with SN2cannot be aggregated together in the same A-MDPU. In other words, thisimplies that, if the PV0 Data frame uses the counter of the SN1, the PV1Data frame is also managed sequentially by using the same counter, i.e.,SN1, so that the frames can be aggregated together in the same A-MDPU.This is because an STA which has received a corresponding A-MPDU assumesthat a sequence number of MPDUs included in the A-MPDU is sequentiallyincreased when transmitting an acknowledgement through Block ACK.

Fourth, all Duration fields of PV0 Data frames constituting the A-MPDUsubframe may be identical. The Duration field is set for the purpose ofprotecting a TXOP duration or a Response PPDU to be transmitted after acorresponding A-MPDU. Other STA does not access the channel during aninterval indicated by the Duration field. In case of the PV1 Data frame,the Duration field may not be included in a MAC header. A Duration fieldvalue in the PV0 Data frame may also indicate a Duration field value ofthe PV1 data frame when the PV1 Data frame is aggregated with the PV1Data frame in the A-MPDU frame.

Fifth, for the PV0 Data frame and PV1 Data frame constituting the A-MPDUsubframe, among Ack Policy fields of corresponding frames, the number of“Normal Ack or Implicit Block Ack requests”, i.e., A-MPDU subframes forrequesting an immediate control response, may not be equal to or greaterthan 2. This is because a collision occurs in a plurality of immediatecontrol responses in this case.

If there is no PV0 Data frame to be transmitted, only the PV1 Data framemay be included in an A-MPDU subframe of an A-MPDU frame. In this case,a TXOP Duration or a Response PPDU to be transmitted at a later timecannot be protected. This is because the Duration field does not existin the MAC header of the PV1 Data frame. In this case, the following PV0Null Data frame can be used.

FIG. 9 shows an A-MPDU format having a PV0 Null Data frame.

A PV0 Null Data frame may include an MPDU not having an MSDU. The PV0Null Data frame may be used to protect a TXOP Duration or a ResponsePPDU to be transmitted at a later time through a duration field of a MACheader.

If only the PV1 Data frame is included as an A-MPDU subframe of theA-MPDU frame or if the PV1 Data frame is transmitted as a single PPDU,there is still a problem in that the TXOP Duration or the Response PPDUto be transmitted at a later time cannot be protected.

As a solution for this, a Response Indication field may be included in aPLCP Header of a corresponding PPDU, for example, in a signal field. Thesignal field may be included in a physical layer preamble of a PPDU. Forexample, the Response Indication field may be in included in L-SIG,HEW-SIGA or HEW-SIGB of an HEW PPDU.

The Response Indication field may indicate a type of an expectedresponse used to protect the response frame. The Response Indicationfield may indicate a type of a Response PPDU to be transmitted after thecorresponding PPDU transmitted at the moment.

The Response Indication field may be set to a value indicating one of NoResponse, Normal Response and Long Response. The No Response indicatesno immediate response that implies that there is no Response PPDU to betransmitted after the corresponding PPDU. The Normal Response indicatesthat an addressed recipient returns an individual control responseframe. The Normal Response may imply that a control response PPDU suchas ACK or Block ACK is to be transmitted starting one Short InterframeSpace (SIFS) after the end of the corresponding PPDU. The Long Responseindicates that an addressed recipient may return a response frame whichis not an individual control response frame. The Long Response may implythat a response PPDU such as a normal DATA PPDU other than ACK and theBlock ACK is to be transmitted starting one SIFS after the end of thecorresponding PPDU.

Hereinafter, it is proposed a channel access scheme when a plurality ofSTAs operate in a Power Save (PS) mode under dense WLAN environments.

A STA operating in the PS mode transitions between an awake state and adoze state. In the awake state, the STA is fully powered. In the dozestate, the STA is not able to transmit or receive and consumes very lowpower. When operating in the PS mode, the STA listens to selected Beaconframes and sends PS-Poll frames to the AP if the TIM element in the mostrecent Beacon frame indicates an individually addressed bufferable unit(BU) is buffered for that STA. The AP transmits buffered individuallyaddressed BUs to the STA only in response to the PS-Poll frame. The STAin the doze state may enter the awake state to receive selected Beaconframes.

An operation of an HEW STA operating in a PS mode is as follows. An STAwhich has transitioned from a doze state to an awake state for frametransmission may perform a CCA process until: 1) a Network AllocationVector (NAV) of the STA is correctly set by detecting a sequence for acertain frame; or 2) a duration corresponding to ProbeDelay elapses.

However, with the use of techniques such as Beam-forming, Multi-channel,MIMO, and OFDMA, it has become more difficult to set an NAV by correctlyreceiving a Duration field in a MAC header of an MPDU. Therefore, it isproposed to perform the CCA process by the HEW STA transitioned from theDoze state to the Awake state until at least one of the followingcondition is satisfied:

-   -   1) a sequence for a certain frame is detected so that an NAV of        the STA is correctly set;    -   2) a signal field of a Physical Layer Convergence Protocol        (PLCP) header is correctly received so that a type of a response        PPDU to be transmitted after a corresponding PPDU is correctly        detected and set through a Response Indication field;    -   3) a duration corresponding to ProbeDelay elapses.

If a newly changed rule is applied to the HEW STA operating in the PSmode, power consumption can be decreased since a channel access canstart when only a signal field of a PLCP header is successfully decoded.The PLCP header may also be called as a physical header.

If a certain STA correctly receives a signal field of a physical headerand thus correctly sets a type of a Response frame transmitted after acorresponding PPDU with a Response Indication field, a first intervalcan be utilized to defer a channel access without having to use a secondinterval even if an MPDU of a corresponding PPDU cannot be successfullydecoded and thus a Duration field value cannot be correctly identified,since the signal field is decoded in a physical layer but the MPDU isdecoded in a MAC layer. The type of the Response frame in the receivedPPDU can be identified even when it is only decoded in the physicallayer successfully. The first interval may be shorter than the secondinterval. This is to decrease power consumption by starting a channelaccess in much quicker time. The first interval may be a Distributedcoordination function (DCF) Interframe Space (DIFS) and the secondinterval may be an Extended Interframe space (EIFS).

Interframe space (IFS) is a time interval between frames and is used todefer a channel access. A STA determines whether a wireless medium isbusy or idle through the use of the carrier sense (CS) function. Whenthe wireless medium is busy, the STA defers the access of the mediumduring a DIFS or an EIFS. A STA can determine whether the medium is busywhen a correctly received and decoded a frame. In general, after DIFSexpires, the STA tries to access the medium. A correctly received frameis a frame that has successfully decoded but a STA can determine whetherthe medium is busy when an incorrectly received a frame. In general,after EIFS expires, the STA tries to access the medium. The incorrectlyreceived frame is a frame that has unsuccessfully decoded. In anembodiment, an intermediately received frame is defined. Theintermediately received frame is a frame that has successfully decodedin a physical layer but has unsuccessfully decoded in a MAC layer. Thismeans that a STA can decode a signal field of the frame and can obtainthe Response Indication field to identify the type of the Responseframe. If the STA receives the intermediately received frame after theSTA transitions from a doze state to an awake state, the STA may deferthe channel access not during the EIFS but during the DIFS. Since theDIFS is shorter than the EIFS, the STA can access the medium faster.

FIG. 10 is a block diagram of an STA according to an embodiment of thepresent invention.

The STA may include a processor 21, a memory 22, and a Radio Frequency(RF) module 23.

The processor 21 implements an operation of the STA according to theembodiment of the present invention. The processor 21 may generate aPPDU according to an embodiment of the present invention and mayinstruct the RF module 23 to transmit the PPDU. The memory 22 storesinstructions for the operation of the processor 21. The storedinstructions may be executed by the processor 21 and may be implementedto perform the aforementioned operation of the STA. The RF module 23transmits and receives a radio signal.

The processor may include Application-Specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

In the above exemplary systems, although the methods have been describedon the basis of the flowcharts using a series of the steps or blocks,the present invention is not limited to the sequence of the steps, andsome of the steps may be performed at different sequences from theremaining steps or may be performed simultaneously with the remainingsteps. Furthermore, those skilled in the art will understand that thesteps shown in the flowcharts are not exclusive and may include othersteps or one or more steps of the flowcharts may be deleted withoutaffecting the scope of the present invention.

What is claimed is:
 1. A method for operating in a power save mode for awireless local area network, the method comprising: transitioning, by astation, from a doze state to an awake state; receiving, by the stationin the awake state, a frame via a wireless medium; and, determining, bythe station in the awake state, an Interframe space (IFS) to defer anaccess of the wireless medium when medium access control (MAC) layerdecoding of the frame is unsuccessful and physical layer decoding of theframe is successful; wherein the IFS is set to a first interval when aprotected duration is identified within successfully decoded portion ofthe frame; wherein the IFS is set to a second interval when theprotected duration is not identified within successfully decoded portionof the frame; and wherein the second interval is greater than the firstinterval.
 2. The method of claim 1, wherein the second interval iscalculated based on an Extended Interframe space (EIFS).
 3. The methodof claim 1, wherein the first interval is calculated based on aDistributed coordination function (DCF) Interframe Space (DIFS).
 4. Themethod of claim 1, wherein the successfully decoded portion of the frameis a signal field in a physical header of the frame.
 5. A deviceconfigured for operating in a power save mode for a wireless local areanetwork, the device comprising: a radio frequency module configured totransmit and receive radio signals; a processor operatively coupled withthe radio frequency module; and memory disposed to said processor, saidmemory including instructions that, when executed by said processor,causes the device to: transition from a doze state to an awake state;receive a frame via a wireless medium; and determine an Interframe space(IFS) to defer an access of the wireless medium when decoding of theframe in a medium access control (MAC) layer is unsuccessful anddecoding of the frame in a physical layer is successful, wherein the IFSis set to a first interval when a protected duration is identifiedwithin successfully decoded portion of the frame; wherein the IFS is setto a second interval when the protected duration is not identifiedwithin successfully decoded portion of the frame; and wherein the secondinterval is greater than the first interval.
 6. The device of claim 5,wherein the second interval is calculated based on an ExtendedInterframe space (EIFS).
 7. The device of claim 5, wherein the firstinterval is calculated based on a Distributed coordination function(DCF) Interframe Space (DIFS).
 8. The device of claim 5, wherein thesuccessfully decoded portion of the frame is a signal field in aphysical header of the frame.